IR absorbing dyes have numerous applications, such as optical recording systems, thermal writing displays, laser filters, infrared photography, medical applications and printing. Typically, it is desirable for the dyes used in these applications to have strong absorption in the near-IR at the emission wavelengths of semiconductor lasers (e.g. between about 700 and 2000 nm, preferably between about 700 and 1000 nm). In optical recording technology, for example, gallium aluminium arsenide (GaAlAs) and indium phosphide (InP) diode lasers are widely used as light sources.
Another important application of IR dyes is in inks, such as printing inks. The storage and retrieval of digital information in printed form is particularly important. A familiar example of this technology is the use of printed, scannable bar codes. Bar codes are typically printed onto tags or labels associated with a particular product and contain information about the product, such as its identity, price etc. Bar codes are usually printed in lines of visible black ink, and detected using visible light from a scanner. The scanner typically comprises an LED or laser (e.g. a HeNe laser, which emits light at 633 nm) light source and a photocell for detecting reflected light. Black dyes suitable for use in barcode inks are described in, for example, WO03/074613.
However, in other applications of this technology (e.g. security tagging) it is desirable to have a barcode, or other intelligible marking, printed with an ink that is invisible to the unaided eye, but which can be detected under UV or IR light.
An especially important application of detectable invisible ink is in automatic identification systems, and especially “netpage” and “Hyperlabel™” systems. Netpage systems are described in the following patent applications, all of which are incorporated herein by reference.
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In general, the netpage system relies on the production of, and human interaction with, netpages. These are pages of text, graphics and images printed on ordinary paper, but which work like interactive web pages. Information is encoded on each page using ink which is substantially invisible to the unaided human eye. The ink, however, and thereby the coded data, can be sensed by an optically imaging pen and transmitted to the netpage system.
Active buttons and hyperlinks on each page may be clicked with the pen to request information from the network or to signal preferences to a network server. In some forms, text written by hand on a netpage may be automatically recognized and converted to computer text in the netpage system, allowing forms to be filled in. In other forms, signatures recorded on a netpage may be automatically verified, allowing e-commerce transactions to be securely authorized.
Netpages are the foundation on which a netpage network is built. They may provide a paper-based user interface to published information and interactive services.
A netpage consists of a printed page (or other surface region) invisibly tagged with references to an online description of the page. The online page description is maintained persistently by a netpage page server. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the input elements on the page, including buttons, hyperlinks, and input fields. A netpage allows markings made with a netpage pen on its surface to be simultaneously captured and processed by the netpage system.
Multiple netpages can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage is assigned a unique page identifier. This page ID has sufficient precision to distinguish between a very large number of netpages.
Each reference to the page description is encoded in a printed tag. The tag identifies the unique page on which it appears, and thereby indirectly identifies the page description. The tag also identifies its own position on the page.
Tags are printed in infrared-absorptive ink on any substrate which is infrared-reflective, such as ordinary paper. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter.
A tag is sensed by an area image sensor in the netpage pen, and the tag data is transmitted to the netpage system via the nearest netpage printer. The pen is wireless and communicates with the netpage printer via a short-range radio link. Tags are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless. Tags are error-correctably encoded to make them partially tolerant to surface damage.
The netpage page server maintains a unique page instance for each printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in the page description for each printed netpage.
Hyperlabels™ is a trade mark of Silverbrook Research Pty Ltd, Australia. In general, Hyperlabel™ systems use an invisible (e.g. infrared) tagging scheme to uniquely identify a product item. This has the significant advantage that it allows the entire surface of a product to be tagged, or a significant portion thereof, without impinging on the graphic design of the product's packaging or labeling. If the entire surface of a product is tagged (“omnitagged”), then the orientation of the product does not affect its ability to be scanned i.e. a significant part of the line-of-sight disadvantage of visible barcodes is eliminated. Furthermore, if the tags are compact and massively replicated (“omnitags”), then label damage no longer prevents scanning.
Thus, hyperlabelling consists of covering a large portion of the surface of a product with optically-readable invisible tags. When the tags utilize reflection or absorption in the infrared spectrum, they are referred to as infrared identification (IRID) tags. Each Hyperlabel™ tag uniquely identifies the product on which it appears. The tag may directly encode the product code of the item, or it may encode a surrogate ID which in turn identifies the product code via a database lookup. Each tag also optionally identifies its own position on the surface of the product item, to provide the downstream consumer benefits of netpage interactivity.
Hyperlabels™ are applied during product manufacture and/or packaging using digital printers, preferably inkjet printers. These may be add-on infrared printers, which print the tags after the text and graphics have been printed by other means, or integrated colour and infrared printers which print the tags, text and graphics simultaneously.
Hyperlabels™ can be detected using similar technology to barcodes, except using a light source having an appropriate near-IR frequency. The light source may be a laser (e.g. a GaAlAs laser, which emits light at 830 nm) or it may be an LED.
From the foregoing, it will be readily apparent that invisible IR detectable inks are an important component of netpage and Hyperlabel™ systems. In order for an IR absorbing ink to function satisfactorily in these systems, it should ideally meet a number of criteria:                (i) compatibility with inkjet printers;        (ii) compatibility of the IR dye with aqueous solvents used in inkjet inks;        (iii) intense absorption in the near infra-red region (e.g. 700 to 1000 nm);        (iv) zero or low intensity visible absorption;        (v) lightfastness;        (vi) thermal stability;        (vii) zero or low toxicity;        (viii) low-cost manufacture;        (ix) adheres well to paper and other media; and        (x) no strikethrough and minimal bleeding of the ink on printing.        
Hence, it would be desirable to develop IR dyes and ink compositions fulfilling at least some and preferably all of the above criteria. Such inks are desirable to complement netpage and Hyperlabel™ systems.
Some IR dyes are commercially available from various sources, such as Epolin Products, Avecia Inks and H.W. Sands Corp.
In addition, the prior art describes various IR dyes. U.S. Pat. No. 5,460,646, for example, describes an infrared printing ink comprising a colorant, a vehicle and a solvent, wherein the colorant is a silicon (IV) 2,3-naphthalocyanine bis-trialkylsilyloxide.
U.S. Pat. No. 5,282,894 describes a solvent-based printing ink comprising a metal-free phthalocyanine, a complexed phthalocyanine, a metal-free naphthalocyanine, a complexed naphthalocyanine, a nickel dithiolene, an aminium compound, a methine compound or an azulenesquaric acid.
However, none of the prior art dyes can be formulated into ink compositions suitable for use in netpage or Hyperlabel™ systems. In particular, commercially available and/or prior art inks suffer from one or more of the following problems: absorption at wavelengths unsuitable for detection by near-IR sensors; poor solubility or dispersibility in aqueous solvent systems; or unacceptably high absorption in the visible part of the spectrum.
In a typical netpage, there may be a large number of hyperlinks on one page and correspondingly relatively large areas of the page printed with IR ink. In the Hyperlabel™ system, the majority of a product's packaging may be printed with the invisible ink. Thus, it is especially desirable that the ink used is invisible to the unaided eye and contains minimal residual colour.
Moreover, inkjet printing is the preferred means for generating netpages and Hyperlabel™. Ink-jet printing is preferred primarily for its high-speed and low cost. Inkjet inks are typically water-based for reasons of low cost, low toxicity and low flammability. In thermal bubble-jet printers, the ink needs to be rapidly vaporized during the printing process. This rapid vaporization of the ink during the printing process necessitates a water-based ink composition. Accordingly, it is desirable that the IR dyes used in netpage and Hyperlabel™ inks are suitable for formulating into aqueous ink compositions and are compatible with inkjet printers.
A further essential requirement of IR dyes used in netpage systems is that they must absorb IR radiation at a frequency complementary to the frequency of the IR sensor in the netpage pen. Preferably, the ink should contain a dye, which absorbs strongly at the frequency of the IR sensor. Accordingly, the dyes used in netpage systems should absorb strongly in the near-IR region—that is, 700 to 1000 nm, preferably 750 to 900 nm, more preferably 780 to 850 nm.